An electrodeless discharge lamp does not need electrodes in the lamp bulb as it is ignited with applying a microwave out of a magnetron. Therefore, its operation life is long because of no decay of electrodes, no contamination of enclosed gas and no contamination of lamp envelope of quartz glass or so. The focus of the electrodeless discharge lamp is smaller than an electroded discharge lamp because of its small inner diameter. The electrodeless discharge lamp should be held in an adequate microwave cavity resonator and the magnetron microwave should be applied through an adequate waveguide (guide tube or coaxial cable) to the lamp with an antenna situated in the cavity resonator in order to apply microwave power to the electrodeless discharge lamp efficiently.
Several thousand volts and several hundred milliamperes are required to drive a magnetron. The magnetron is ordinarily driven by rippling power rectified from commercial frequency power. A power source device of this kind has advantages of low cost manufacturing and few breakdowns because of its simple structure. And also, magnetron drive power can be controlled according to required illumination in a range from 35% to 100% of full power.
The electrodeless discharge lamp ignited by a magnetron microwave is utilized at film processing in FPD industry and wiredraw process of optical fibers and also UV coating. Coating material used in these processes is generally called photosensitive resin (photopolymer). Photosensitive resin will solidify, dry or harden with irradiation of more than certain quantity of UV radiation. And also, UV radiation irradiation can raise the removability or adhesion. The processes of low-energy consumption and less dissolvent consumption are required for ecological protection (e.g., exhaust control of CO2 or VOC). The process using photosensitive resin does not need dissolvent basically. Photosensitive resin is processed with UV radiation instead of heat. UV process needs very little energy consumption and dissolvent consumption compared to heat processing.
In recent years, there is a strong requirement to raise manufacturing line speed. And also, there is a strong requirement for stability to manufacture stable products. The speed of wiredraw line of optical fiber is 1000 m/min. The film processing speed is from 100 to 200 m/min. Uniform products can hardly be yielded because of local fluctuation of UV radiation irradiation and high speed of the manufacturing line in case of using rippled power source in these high-speed processes. The discharge lamp blinks at an interval from about 8 msec to 10 msec according to a frequency of 50 Hz or 60 Hz and gives rise to no-irradiation periods of UV radiation. Then inequality of irradiation on products is caused and uniform products are not available. And also, the discharge lamp lifetime is shortened because blinking is repeated twice at 50 Hz or 60 Hz in frequency.
Then, DC power is introduced to drive a magnetron. The magnetron can be driven with DC power of fewer ripples applying pulse-mode switching power supply. Consequently, microwave power intensity becomes stable. Irradiation becomes constant and good quality light can be available when such power is applied to the discharge lamp. When the discharge lamp is driven by a conventional ripple power supply at the mean power equal to DC drive, the peak output of such a magnetron is larger than that of the DC driven magnetron by reason of the long stop period of the magnetron driven by the ripple power supply. Consequently, the maximum peak power of the magnetron becomes large and the magnetron lifetime is shortened. And the microwave input to the discharge lamp is in ripple state. The maximum peak input of microwave is large. The discharge lamp lifetime is shortened. The lifetime of the magnetron and the discharge lamp is extended when the lamp is driven with ripple-free DC power supply.
DC driven magnetron has advantages as above. On the other hand, there is also a fault that abnormal oscillation tends to occur. An abnormal state may occur suddenly even if magnetron drive power is stable and a discharge lamp is also at steady state when ripple-free DC power is applied. A magnetron oscillation frequency and a magnetron output are varying according to reflective wave amplitude and phase. This situation is expressed with a Rieke diagram for each magnetron. When sudden abnormal operation arises in a magnetron, the magnetron does not return to the normal state of stable operation because an anode voltage does not reach zero since DC power is supplied. This is not a desirable state for a magnetron and a discharge lamp. In the case of a UV radiation discharge lamp used in a production line, it is needed to suppress discontinuation of working by rapid returning to the steady state as much as possible at the early stage after occurrence of the abnormal state. It is also demanded to return to the stable luminescence state before damaging the discharge lamp. To avoid the abnormal oscillation of a magnetron, high-voltage magnetron drive power is stopped periodically or a magnetron is stopped temporarily when an abnormal state is detected.
And, a magnetron heats up itself by reflective waves and it damages if the luminescence medium in the microwave excitation electrodeless discharge lamp evaporates inadequately right after the lighting start. In order to prevent this, the starting period after the lighting start is extended longer than the time for the luminescence medium to absorb microwave and to evaporate fully. That is, the magnetron is driven with ripple-free DC power, and at the start period immediately after lighting start, the magnetron power is increased according with evaporation of luminescence medium in order to ensure the longer starting period than that for the luminescence medium to absorb microwave and fully evaporate. This is known as a soft start of an electrodeless discharge lamp system. Some examples of conventional technology relevant to the solution for abnormal oscillation of a magnetron are cited as below.
A high-frequency heating apparatus as disclosed in the patent document 1 is that to prevent the damage of a magnetron and the other constituent parts due to unstable oscillation or moding, and at the same time to increase reliability of detecting the unstable oscillation. At the time of moding of a magnetron, it is detected by a current detection circuit to cut off the ON signal of an inverter circuit by a control circuit. And when heating is started, control based on an input from a current detection circuit is stopped for a certain period of time or a magnetron current detecting function is stopped until the level value of a current feedback means reaches a certain value.
Detection of abnormality of a magnetron as disclosed in the patent document 2 is a method to detect the abnormal oscillation of a magnetron with regard to a high frequency heater using the magnetron energized by an inverter power source. An input current detection means detects the current input into an inverter power source. An abnormal oscillation detection means detects moding, i.e. abnormal action of a magnetron. Abnormal oscillation of the magnetron can be detected by comparing the current input into the inverter power source and the reference value of the abnormal oscillation detection means with one another.
An electrodeless lamp system as disclosed in the patent document 3 is that to prevent the break due to self-heating of a magnetron by a reflected wave. By an electromagnetic field of a microwave generated by a magnetron, a luminescence medium sealed inside the electrodeless lamp is excited to emit light. This is provided with a soft starter means in which an electric power to drive the magnetron is gradually increased. This soft starter means is that which is to prevent the break due to the self-heating of the magnetron by the reflected wave, and used at the start of the light-emission of the electrodeless lamp.
A high-frequency heating device as disclosed in the patent document 4 is that to prevent a magnetron from continuing actions in a state of moding oscillation, and prevent lifetime deterioration of the magnetron. A moding oscillation detecting means detects that the magnetron is making the moding oscillation. An inverter circuit is stopped or restarted based on this information. Using the moding oscillation detecting means, the continuation of the moding oscillation of the magnetron can be prevented. A high-frequency heating device with a DC power supply such as a battery can be realized without damaging the lifetime of the magnetron.
A magnetron driving power source device as disclosed in the patent document is that to drive a magnetron by using a simple circuit configuration without installing an abnormality detecting circuit. A high voltage generating means generates a high voltage to drives the magnetron. A stop signal generating means generates a stop signal to stop the operation of the high voltage generating means at predetermined intervals only for a short period when the magnetron can be stopped. The high voltage generating means stops high voltage generation in response to the stop signal from the stop signal generating means.    [Patent document 1] JP05-251174A (1993)    [Patent document 2] JP07-014672A (1995)    [Patent document 3] JP2003-068490A    [Patent document 4] JP2003-100440A    [Patent document 5] JP2004-200051A